Digital Subscriber Line (DSL) technology, including e.g. ADSL, ADSL2, (S)HDSL, VDSL, VDSL2 up to the upcoming G.fast, during all its history, attempted to increase the bit rate in the aim to deliver more broadband services to the customer. Unfortunately, copper loops deployed from a Central Office (CO) to customer premises (CPE) are rather long and do not allow transmission of data with bit rates more than few Mb/s. Therefore, to increase the customer available bit rates, modern access networks use street cabinets, MDU-cabinets, and similar arrangements, also referred to as distribution points (DP): the cabinet or other DP is connected to the CO by a high-speed fiber communication line, e.g., gigabit passive optical network (GPON) and installed close to the customer premises. From these cabinets, high-speed DSL systems, such as Very-High-Bit-Rate DSL (VDSL), provide connection to the CPE. The currently deployed VDSL systems (ITU-T Recommendation G.993.2) have range of about 1 km, providing bit rates in the range of tens of Mb/s. To increase the bit rate of VDSL systems deployed from the cabinet, the recent ITU-T Recommendation G.993.5 defined vectored transmission that allows increasing upstream and downstream bit rates up to 100 Mb/s and more. Vectoring will also be used in upcoming G.fast.
One important component or stage of DSL systems is initialization (or training). During the initialization, channels that join to the vectored group provide the ability for existing active channels to accommodate crosstalk from new channels, provide the ability for joining channels to accommodate crosstalk from active channels and other joining channels, and finally provides joining channels with proper transmit power and bit loading.
This application addresses, amongst others, initialization and adaptation of vectored channels. One serious issue with vectored channels is high crosstalk, especially when very high frequencies (such as 5 MHz and higher) are used. During initialization and training, when FEXT (far-end crosstalk) between channels established on lines of a cable binder comprising a plurality of lines is not reduced or cancelled, signals transmitted over channels are “visible” in all other channels. FEXT can be the dominant disturber of data transmission. Generally, it is possible to cancel FEXT at the CO-side by vectoring.
Typically, in downstream direction, FEXT can be cancelled by pre-coding transmit signals sent on the channel. In upstream direction, FEXT can be cancelled by post-processing signals received on the channels. In both cases, typically, the vectoring processor (VP) needs to have access to the signals of all channels in the cable binder. Cancellation is usually done in frequency domain by weighting transmit and receive symbols of all channels by a so-called cancellation matrix in downstream direction and upstream direction, respectively. The cancellation matrix thus describes the FEXT between any two channels of lines of a cable binder.
The cancellation matrix can be calculated, e.g., during initialization, by means of parameters obtained from channel estimation. Generally, it is possible that the VP either estimates the channels directly and calculates the cancellation matrix based on the channel estimation, or uses values provided by the central office and the CPE in order to calculate or adapt the cancellation matrix. Usually, the crosstalk parameters are adapted after initialization has finished during Showtime, e.g., by means of an adaptive algorithm. Then, the cancellation matrix is updated/adapted accordingly.
Usually, for channel estimation synchronization symbols are included in a stream or sequence of symbols transmitted via the channel. Sometimes, a situation may occur where one or more synchronization symbol are significantly affected by non-FEXT noise present on the channel, e.g., background noise or impulse noise. If, in such a scenario, a synchronization symbol is used to determine/adapt the cancellation matrix, this may result in a reduced accuracy of FEXT reduction. In particular, impulse noise present on the channel may have a significant impact on the accuracy with which the cancellation matrix is determined.
To address this issue to some degree, it is known to provide a so-called reliability bit in a message that is used by the CPE to report the error vector, see ITU-T Rec. G.993.5, Section 7.2.3.1. The reliability bit seeks to indicate whether the reported error values are reliable or not. However, the usage of such the reliability bit may be inaccurate and it may be questionable whether the reliability bit has been determined accurately. Also, generation of the reliability bit resides within the duty of the CPE which can increase control signalling and increase inaccuracies in determining the reliability bit.
It is also known to estimate impose noise based on reported error of feedback values, see US 2012/0106605 A1. However, also such approaches are comparably sensitive to background noise, in particular impulse noise.